The present invention relates to a method for processing and recirculating filtration residues.
When tars, pitches, coal extracts and coal oils are filtered, it is known to improve filterability by employing a filter aid. This is deposited as an alluvial layer on the actual filter structure and/or it is constantly added in metered quantities to the substance to be filtered. In either case, it will form on the actual filter structure a layer through which the substance if filtered before it even encounters the actual filter structure per se.
In the course of continuing filtration, a filter cake (filter residue) builds up which is primarily composed of the filter aid, the filtered-out substances and any residual filtrate that clings to them. From time to time this filter cake is (in fact, must be) removed from the actual filter structure. Either prior to such removal, or subsequently thereto, it is customary to dry the filter cake to a certain extent by means of an inert gas, such as N.sub.2 or hot steam. This reduces the amount of residual filtrate adhering to the filter cake and permits recovery of the portion which is driven off by the influence of the heat or gas.
Among the suitable filter aids are perlite, swelling clay and kieselguhr. Both of these, due to their particular structure, form a loose (as opposed to dense) filter cake, which assures high flow-through rates per unit time and a high degree of filtrant purity. Under actual process conditions it is customary to match the residue quantities to be removed by filtration, with the addition of similar quantities of filter aid. Another type of suitable filter aid is swelling or bubble coke; these have similarly advantageous characteristics as perlite, swelling clay and kieselguhr but are used preferentially for the filtration of alkaline media. These coke types are formed by subjecting finely ground bituminous products to sudden heating. During this thermal treatment the material passes through a softening phase; at the same time, gas evolution begins to take place. This gas expands (blows up) the individual granules to form a hollow, thin-walled product (bubble).
There are two major problems involved in the filtration of tars, pitches, coal extracts and coal oils: the quantities of (relatively expensive) filter aid which are needed are very substantial and little, if any, of the filter aid can be recovered for re-use. The other problem is what to do with the filter cake for which no reasonable use exists.
The latter problem could be dealt with according to a prior art proposal which suggests that the filtration residue should be simply combusted. However, this evidently does not solve the problem of providing large quantities of fresh filter aid for each filtration episode.